An in-core instrument (ICI) is a device for measuring the output of a nuclear reactor by measuring the density and temperature of neutron flux in a core of the nuclear reactor.
In the related art, in-core instruments were inserted into a core through the bottom of a reactor vessel, but there was a problem that the substances in the core of a reactor may leak through the hole formed through the bottom of the reactor vessel.
In order to solve this problem, all in-core instruments have been disposed close to a core through a hole at the top of a reactor vessel instead of the way of inserting them through the bottom of a reactor vessel.
In-core instruments inserted through the closure head reach the inside the nuclear fuel assembly through instrumentation tubes and their ends block cooling holes in the lower end fitting for the nuclear fuel assembly, depending on the insertion positions, so the flow rate of cooling water that flows into the instrumentation tubes may not be sufficiently secured.